The present application pertains generally to audio scramblers, and pertains more particularly to scramblers intended for use with pay television or similar systems.
With the advent of large scale cable TV and multipoint distribution systems, it has become increasingly important to prevent the unauthorized use of such systems. The usual method for doing so is to scramble at least part of the signal provided to the system's subscribers. It is most common to scramble the video signal. This, however, has proved not always sufficient, since some unauthorized users find the audio satisfactory by itself, even if the video is unusable. Some systems which scramble the audio signal instead of, or in addtion to, the video signal are known. Such systems, however, are usually relatively complex and expensive. Many such systems involve dividing the audio signal into several frequency subbands and scrambling them. This approach is quite costly. Another approach involves scrambling one or both signals in accordance with a simple pattern built into both the transmitter and a unit installed in the subscriber's home. Unfortunately, it has often proved relatively easy for an enterprising unauthorized person to identify the pattern and build a reception unit that will serve as well as one provided by the broadcaster. Still another approach has been to add a random noise signal or interfering signal in random phase to the audio, and to broadcast the noise on a separate channel, or subcarrier, the subscriber's unit substracting the noise signal from the audio. A variation of this approach involves adding a random-phase noise or interfering signal to the audio signal only at random or pseudo-random times, and sending an additional signal to cause the reception unit to substract the noise signal from the scrambled audio signal at the proper times. It has also been proposed to provide such information of an unused portion of the video signal, for example, the back porch. This, however, is somewhat cumbersome.
It is the principal object of the invention to provide a simple and economical audio scrambler system and decoder exempt from the above described shortcomings and well suited for use on a large scale in a pay television or similar system.
Another object of the invention is to provide such a system in which the audio scrambling is entirely independent of the video signal, so that the system of the invention can be used either with or without any selected video scrambling system.
Still another object of the invention is such an audio scrambling system in which the scrambling involves frequent and effectively unpredictable changes, to hamper unauthorized unscrambling.
Still another object of the invention is to provide an audio scrambler in which information, preferably coded, used to control unscrambling is transmitted on the same subcarrier as a noise signal used in scrambling, or on a separate subcarrier, or within a video signal transmitted with the audio signal.
Yet another object of the invention is to provide an audio scrambling system fulfilling the above objects, and enabling the broadcaster to address individual decoders to specify the type or types of service the owner of the addressed decoder is authorized to receive, and to disable the addressed decoder if service to its owner is to be suspended.